Chapter 129. Staphylococcal Infections (Part 7)

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Respiratory Tract Infections Respiratory tract infections caused by S. aureus occur in selected clinical settings. S. aureus is a cause of serious infections in newborns and infants; these infections present as shortness of breath, fever, and respiratory failure. Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities). Pneumothorax and empyema are recognized complications of this infection. In adults, nosocomial S. aureus pulmonary infections are commonly seen in intubated patients in intensive care units. The clinical presentation is no different from that encountered in pulmonary infections of other bacterial etiologies. Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and...

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Chapter 129. Staphylococcal Infections (Part 7) Respiratory Tract Infections Respiratory tract infections caused by S. aureus occur in selected clinical settings. S. aureus is a cause of serious infections in newborns and infants; these infections present as shortness of breath, fever, and respiratory failure. Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities). Pneumothorax and empyema are recognized complications of this infection. In adults, nosocomial S. aureus pulmonary infections are commonly seen in intubated patients in intensive care units. The clinical presentation is no different from that encountered in pulmonary infections of other bacterial etiologies. Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates. Distinguishing bacterial pneumonia from respiratory failure of other causes or new pulmonary infiltrates in critically ill
patients is often difficult and relies on a constellation of clinical, radiologic, and laboratory findings. Community-acquired respiratory tract infections due to S. aureus most commonly follow viral infections or septic pulmonary emboli (e.g., in injection drug users). Influenza is the most common cause of the former type of presentation. Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates. Diagnosis is made by sputum Gram's stain and culture. Blood cultures, although useful, are usually negative. Bacteremia, Sepsis, and Infective Endocarditis S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding (establishment of suppurative collections at other tissue sites). The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%. Among the more commonly seeded tissue sites are bones, joints, kidneys, and lungs. Recognition of these complications by clinical and laboratory diagnostic methods alone is often difficult. Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency. Other host factors associated with an increased risk of complications include presentation with
community-acquired S. aureus bacteremia (except in injection drug users), lack of an identifiable primary focus, and the presence of prosthetic devices or material. Clinically, S. aureus sepsis presents in a manner similar to that documented for sepsis due to other bacteria. The well-described progression of hemodynamic changes—beginning with respiratory alkalosis and clinical findings of hypotension and fever—is commonly seen. The microbiologic diagnosis is established by positive blood cultures. The overall incidence of S. aureus endocarditis has increased over the past 20 years. S. aureus is now the leading cause of endocarditis worldwide, accounting for 25–35% of cases. This increase is due, at least in part, to the increased use of intravascular devices; transesophageal echocardiography (TEE) studies found an infective endocarditis incidence of 25% among patients with S. aureus bacteremia and intravascular catheters. Other factors associated with an increased risk of endocarditis are injection drug use, hemodialysis, the presence of intravascular prosthetic devices, and immunosuppression. Despite the availability of effective antibiotics, mortality rates from these infections continue to range from 20 to 40%, depending on both the host and the nature of the infection. Complications of S. aureus endocarditis include cardiac valvular insufficiency, peripheral emboli, metastatic seeding, and central nervous system (CNS) involvement.
S. aureus is now a leading cause of endocarditis in many countries. S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use, (2) left-sided native-valve endocarditis, (3) prosthetic-valve endocarditis, and (4) nosocomial endocarditis. In each of these settings, the diagnosis is established by recognition of clinical stigmata suggestive of endocarditis. These findings include cardiac manifestations, such as new or changing cardiac valvular murmurs; cutaneous evidence, such as vasculitic lesions, Osler's nodes, or Janeway lesions; evidence of right- or left- sided embolic disease; and a history suggesting a risk for S. aureus bacteremia. In the absence of antecedent antibiotic therapy, blood cultures are almost uniformly positive. Transthoracic echocardiography, while less sensitive than TEE, is less invasive and often establishes the presence of valvular vegetations. Acute right-sided tricuspid valvular S. aureus endocarditis is most often seen in injection drug users. The classic presentation includes a high fever, a toxic clinical appearance, pleuritic chest pain, and the production of purulent (sometimes bloody) sputum. Chest x-rays reveal evidence of septic pulmonary emboli (small, peripheral, circular lesions that may cavitate with time). A high percentage of affected patients have no history of antecedent valvular damage. At the outset of their illness, patients may present with fever alone, without cardiac or other localizing findings. As a result, a high index of clinical suspicion is essential to the diagnosis.
Individuals with antecedent cardiac valvular damage more commonly present with left-sided native-valve endocarditis involving the previously affected valve. These patients tend to be older than those with right-sided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, and metastatic seeding) is higher. S. aureus is one of the more common causes of prosthetic-valve endocarditis. This infection is especially fulminant in the early postoperative period and is associated with a high mortality rate. In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary. Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation. The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices. This form of endocarditis is most commonly caused by S. aureus. Because patients often are critically ill, are receiving antibiotics for various other indications, and have comorbid conditions, the diagnosis is not easily recognized.